1. Field of the Invention
The present invention relates to a light-emitting device. More particularly, the invention relates to an active matrix type light-emitting device having thin film transistors (TFTs) over an insulator.
2. Related Art
In recent years, the technique of forming TFTs over a substrate has made drastic progress to develop the applications to the active matrix type display device (or the light-emitting device). Especially, the TFTs using a poly-silicon film have a higher field effect mobility (or simply, a mobility) than that of the TFTs using an amorphous silicon film of the prior art so that they can act at a high speed. Therefore, the control of the pixels, as has been made in the prior art by a driver circuit outside of the substrate, can be made by a driver circuit which is formed over the substrate common to the pixels.
This active matrix type light-emitting device is enabled by forming various circuits and elements over the common substrate to have various advantages such as the reduction in the manufacture cost, the size reduction of an electro-optic device, the rise in the yield or the drop of the throughput.
Moreover, there has been vigorously investigated the active matrix type light-emitting device (or the EL display) which has EL elements as its light-emitting elements. The EL display is also called either the organic EL display (OELD) or the organic light emitting diode (OLED).
The EL display is of the spontaneous luminescence type. The EL element has a structure in which an EL layer is sandwiched between a pair of electrodes (i.e., an anode and a cathode), and the EL layer usually has a laminated structure, as represented by the structure of “hole transfer layer luminescent layer/electron transfer layer” proposed by Tang et al., of Kodak Eastman Company. This structure has such a high luminescence efficiency that most of the EL displays being investigated and developed adopt it.
The structure may be exemplified by another lamination of hole injection layer/hole transfer layer/luminescence layer/electron transfer layer, or hole injection layer/hole transfer layer/luminescence layer/electron transfer layer/electron injection layer over the anode. The luminescence layer may also be doped with a fluorescent pigment or the like.
Herein, all the layers to be interposed between the cathode and the anode will be generally called the “EL layer”. Specifically, the EL layer means the layer which contains an organic EL material capable of establishing the EL (Electro Luminescence, as established by applying an electric field), and all the aforementioned hole injection layer, hole transfer layer, luminescence layer, electron transfer layer and electron injection layer are contained in the EL layer.
On the other hand, the luminescence to be obtained by the organic EL material is one (of fluorescence) at the return from a single excited state to the ground state or the other (of phosphorescence) at the return from the triple excited state to the ground state. The light-emitting device of the invention can adopt the EL element having either of the organic EL materials.
By applying a predetermined voltage to the EL layer having the aforementioned structure from the paired electrodes, moreover, the carriers in the luminescence layer are recombined to emit light. Herein, the light-emitting element, as formed of the anode, the EL layer and the cathode, will be called the EL element.
In the EL display, there are formed in a matrix shape a plurality of pixels, each of which has a thin film transistor (TFT) and an EL element. FIG. 17 shows a pixel of the EL display in an enlarged scale. A pixel 1700 is composed of a switching TFT 1701, a current controlling TFT 1702, an EL element 1703, a source signal line 1704, a gate signal line 1705, a current supply line 1706 and a capacitor 1707.
A gate electrode of the switching TFT 1701 is connected with the gate signal line 1705. On the other hand, one of the source region and the drain region of the switching TFT 1701 is connected with the source signal line, the other is connected with the gate electrode of the current controlling TFT 1702. A source region of the current controlling TFT 1702 is connected with the current supply line 1706 and a drain region of the current controlling TFT 1702 is connected with the anode or cathode of the EL element 1703.
Where the anode of the EL element 1703 is connected with the drain region of the current controlling TFT 1702, its anode is a pixel electrode, and its cathode is an opposed electrode. Where the cathode of the EL element 1703 is connected with the drain region of the current controlling TFT 1702, on the contrary, its anode is an opposed electrode, and its cathode is a pixel electrode.
Herein, the potential difference between the potential of the pixel electrode and the potential of the opposed electrode will be called the “EL driving voltage”, which is applied to the EL layer.
Here, the capacitor 1707 need not always be provided. If any, the capacitor 1707 is connected with the current controlling TFT 1702 and the current supply line 1706, as shown in FIG. 17.
The potential (i.e., the supply potential) of the current supply line 1706 is held constant. The potential of the opposed electrode of the EL element 1703 is also held constant. This potential of the opposed electrode is given such a potential difference from the supply potential that the EL element may luminesce when the supply potential is applied to the pixel electrode of the EL element.
The switching TFT 1701 is turned ON with the selection signal inputted to the gate signal line 1705. Herein, the ON state of the TFT means that the drain current of the TFT takes a value of 0 or higher.
When the switching TFT 1701 is turned ON, the video signals, as inputted from the source signal line 1704, are inputted through the switching TFT 1701 to the gate electrode of the current controlling TFT 1702. Here, the inputting of a signal through the switching TFT 1701 to the gate electrode of the current controlling TFT 1702 means that the signal is inputted through the active layer of the switching TFT 1701 to the gate electrode of the current controlling TFT 1702.
The amount of the current to flow through the channel forming region of the current controlling TFT 1702 is controlled with a gate electrode Vgs or the potential difference between the gate electrode and the source region of the current controlling TFT 1702. Therefore, the potential to be applied to the pixel electrode of the EL element 1703 is determined by the level of the potential of the video signals, as inputted to the gate electrode of the current controlling TFT 1702. By the level of the potential fed to the pixel electrode, moreover, the luminescent luminance (i.e., the luminance of the light emitted by the EL element) of the EL element is controlled. In other words, the EL element 1703 is controlled in its luminance to effect the gradation display by the potential of the video signals inputted to the source signal line 1704.
In recent years, the reduction in the pixel size has been advanced to desire a finer image. This pixel miniaturization has increased the area for forming the TFT and the wiring line in one pixel thereby to reduce the pixel aperture ratio.
In order to achieve a high aperture ratio of each pixel in a regulated pixel size, therefore, it is essential to make an efficient layout of the circuit elements necessary for the circuit construction of the pixels.
In order to realize the active matrix type light-emitting device of the pixel aperture ratio, as described above, there has been desired a novel pixel construction.